The claimed invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to an apparatus and method for optimizing the transfer of data between a host device and a disc drive data storage device through the use of adaptive bi-directional write skip masks to combine multiple sets of write data associated with different write commands into a single combined write operation.
A disc drive is a data storage device used to store digital data. A typical disc drive includes a number of rotatable magnetic recording discs that are axially aligned and mounted to a spindle motor for rotation at a high constant velocity. A corresponding array of read/write heads access fixed sized data blocks (sectors) on tracks of the discs to write data to and to read data from the discs.
Disc drives are provided with servo control circuitry to move the heads to the various tracks, read/write channel circuitry to write data to and read data from the discs, and interface control circuitry to facilitate communication and data transfer with a host device. A disc drive is typically configured to operate in accordance with an industry standard interface protocol, such as Small Computer Systems Interface (SCSI). Communications and data transfers are carried out between host and drive in accordance with this protocol.
Disc drives of the present generation typically accommodate command queuing, which allows multiple input/output (I/O) commands to be received in a command queue and executed by the drive in an order different than that received. SCSI protocols currently support up to 256 pending commands in the command queue. A search strategy is used to execute the commands in an order that will potentially provide the highest transfer rate. For example, if several commands require access to data blocks close to the current position of the heads, and other commands require access to data blocks at distant locations on the discs, the drive may proceed to execute all of the local accesses before moving the heads to the distant locations and accessing the data blocks at the distant locations to minimize seek time (i.e., time spent moving from one track to the next).
The time required for a particular data block to rotate around and reach the head (latency) is an important factor when selecting the execution order, as delays in waiting for the disc to rotate significantly decrease the resulting transfer rate. Selection of the execution order typically includes estimating how much time it would take to reach each of the data blocks associated with the pending access commands based on latency and the time required to perform any necessary head switches and seeks.
A disc drive can typically employ various run-time selectable strategies (parameters) to improve data transfer performance, such as read on arrival (ROA) and read look ahead (RLA). ROA and RLA generally entail reading data blocks and placing the contents into the data buffer even though the host has not specifically requested the data from such data blocks, on the basis that the host may request the data in the near future.
ROA involves performing a seek command to move the head to a destination track on which a target data block resides, and commencing to read the preceding data blocks on the track until the target data block reaches the head. By contrast, RLA involves receiving a command to move to a new target track, but because the target data block is a large angular distance away from the head, the drive delays seeking to the new track and instead maintains the head on the current track and reads additional data blocks on the current track before moving to the destination track and reading the target data block. The foregoing strategies can provide improved performance under certain circumstances, such as when the command stream has a high degree of locality.
Another run-time selectable parameter that can improve data transfer performance is write caching. Write caching involves delaying the writing of data received from the host in favor of execution of other previously requested accesses (as opposed to immediately writing the data upon receipt). Advantages associated with write caching include the fact that more commands are available to choose from during the sorting strategy, which statistically improves overall access times.
However, allowing write data to linger in the buffer presents some disadvantages as well. Besides the risk of loss of data in the event of a power outage or other anomalous condition, the presence of large amounts of accumulated write data in the buffer takes up valuable space that could be utilized for readback data. Also, controller firmware routines typically only allow a maximum aging of any pending write command; thus, a substantial increase in service time can be observed if the interface circuit is forced to service a large number of write commands to purge old write data to the discs.
Accordingly, there is a need for improvements in the art to provide effective control of cached write data in a disc drive to improve data transfer performance.
In accordance with preferred embodiments, a disc drive data storage device is provided with a buffer (first memory space) and a number of rotatable discs (second memory space).
A host device issues access commands to the disc drive from time to time to transfer data between the host device and the discs. Such access commands include write commands to write sets of data (writeback data) to respective logical block addresses (LBAs) defined on the disc surfaces, and read commands to retrieve sets of previously recorded data (readback data) from selected LBAs on the disc surfaces.
A hardware/firmware based interface circuit employs write caching so that the writeback data are temporarily stored in the buffer pending transfer to the discs in accordance with a sort strategy that sorts the pending read and write access commands in an order designed to optimize data transfer performance.
Bi-directional adaptive write skip masks are employed to combine multiple pending sets of writeback data in the buffer into a single write operation. Each write skip mask generally comprises a selected interval of consecutive LBAs into which multiple writeback data sets are combined when the writeback data have associated LBA ranges that are sufficiently close to fit within the mask interval.
Preferably, as the drive receives each new write command, the interface circuit evaluates the newly added set of writeback data for inclusion into an existing mask. If the newly added set of writeback data does not fit within an existing mask, the interface circuit next determines whether the newly added set of writeback data can be combined with another pending set of writeback data to form a mask.
Each mask is characterized as being adaptive and bi-directional. When first and second sets of writeback data are evaluated for possible combination into a mask, the mask interval is placed over the first set of writeback data so that the mask includes a first portion of consecutive LBAs that precedes the first set of writeback data and a second portion of consecutive LBAs that follows the first set of writeback data. The interface circuit determines whether the second set of writeback data will fit within the first and second portions. Preferably, the mask is configured to be slidable as necessary to accommodate the second set of writeback data; that is, the range of the first portion can be increased or decreased (with a corresponding decrease or increase in the range of the second portion) in order to include both the first and second sets of writeback data within the mask interval.
When both of the writeback sets of data can be included within the mask interval, the mask is formed and a single, combined writeback command is formed in a command queue for subsequent execution. Execution of the combined writeback command results in the writing of the respective datasets in an order corresponding to the order in which the datasets appear in the mask. The mask range can be increased as desired to accommodate the addition of additional sets of writeback data.
The use of bi-directional adaptive masks as disclosed herein advantageously reduces computational overhead required to separately sort and select the individual write command nodes associated with different sets of writeback data in the buffer. Also, such masks advantageously allow writeback data from various commands received in descending order or in random order (with respect to LBA sequence) to be handled nominally as efficiently as writeback data received in ascending order.
These and various other features and advantages that characterize the claimed invention will be apparent upon reading the following detailed description and upon review of the associated drawings.